1. Field of the Invention
The present invention relates to a thin film transistor and a semiconductor device using the thin film transistor.
2. Description of the Related Art
A thin film transistor (TFT) can be formed over an inexpensive glass substrate, and can be widely used as a semiconductor element included in an active matrix semiconductor display device. TFT characteristics such as mobility or ON current are determined depending on the crystallinity of a semiconductor film used for a TFT. For example, a mobility of a TFT with a channel formation region formed of an amorphous semiconductor film is about from 0.4 cm2/V·sec to 0.8 cm2/V·sec, while a mobility of a TFT with a channel formation region formed of a polycrystalline semiconductor film is about from several tens of cm2/V·sec to several hundreds of cm2/V·sec. Therefore, the mobility of a TFT using a polycrystalline semiconductor film is two or more digits higher than a TFT using an amorphous semiconductor film, and thus, high ON current is obtained.
However, a grain diameter of a crystal grain included in a polycrystalline semiconductor film ranges from several micrometers to several hundreds of micrometers. Accordingly, a TFT using a polycrystalline semiconductor film has a disadvantage in that variation in mobility and ON current between elements is large because of nonuniform crystallinity in channel formation regions between the elements, though the TFT using a polycrystalline semiconductor film has an advantage in that mobility and ON current are higher than those of a TFT using an amorphous semiconductor film. Consequently, when a polycrystalline semiconductor film is used for a TFT with emphasis on high mobility, variation occurs between elements. In contrast, when an amorphous semiconductor film is used for a TFT with emphasis on the uniformity of mobility between elements, it is difficult to ensure high mobility.
Therefore, a TFT in which a microcrystalline semiconductor film having a crystal grain of about from several nanometers to several hundreds of nanometers is used for a channel formation region has been developed. With the use of TFTs using a microcrystalline semiconductor film, variation in mobility and ON current between elements can be prevented while it is ensured that mobility and ON current are high to some extent.
In Reference 1 (Japanese Published Patent Application No. H4-242724) and Reference 2 (Japanese Published Patent Application No. 2005-49832), a semiconductor display device using a thin film transistor having a microcrystalline semiconductor film as a switching element is described.